This invention relates to time-temperature indicator (TTI) devices for recording time-temperature relationships. In particular, the present invention describes TTI's that record the duration of exposure to a narrow temperature range, in excess of a predetermined temperature, by means of controlled diffusion of dye within a dye-compatible polymer composition. Differentiating the objects of this disclosure from the prior art is the fact that the time period being recorded far exceeds the time required for sterilization of materials by thermal means. Such a TTI fulfills a need in the design of a number of medical devices.
Medical devices historically fell into two distinct categories: (1) reusable and (2) disposable. The economic demands of modern medical practice have lead to devices that are too expensive to be immediately disposable but, nonetheless, have a finite lifetime. The term "resposables" has been given to these devices which during their useful lifetime will undergo multiple sterilization cycles.
The "fitness for use" of this type of equipment can be maintained by incorporating a means of demonstrating the degree of usage. In this way the user has a means of knowing when the "resposable" will no longer function properly and cause damage to the patient. The present invention demonstrates how the number of sterilization cycles experienced by the "resposable" can be measured, which by inference informs the user the degree of service the device has had.
Many prior art TTI devices have been designed for the low temperature conditions (-20.degree. C. to +10.degree. C.) associated with the safe storage of food. A low temperature TTI device incorporating a paper carrier, or wick, is described by U.S. Pat. No. 4,038,873. It discloses a device based on a diffusing medium that liquefies at or above a predetermined temperature.
The liquified diffusing medium is supported by a paper carrier which it impregnates when the TTI is "activated" or made ready for use. Immediately after "activation" the TTI is rapidly brought below the temperature that it is designed to monitor. In this way the liquid in the wick solidifies and no longer functions as a carrier for the indicating material. Failure to rapidly solidify the liquid "keeps the clock running" and leads to false interpretation of the signal.
The indicating material is a dye composition capable of diffusing through the paper-supported diffusing medium, when it liquifies, at a temperature in the range it is designed to monitor.
Another TTI requiring activation is disclosed in U.S. Pat. No. 5,120,137 which describes a reservoir containing an indicating material that liquifies at or above a predetermined temperature. The reservoir is mounted on a base sheet, and a removable barrier is interposed between the reservoir and a wick of very fine interconnected fibers. Removing the barrier activates the TTI by placing the wick in contact with the reservoir, thereby enabling the migration of the indicating material along the wick at a controlled rate upon liquification.
There exists a need for a TTI that does not need to be "activated" but is ready for use without user intervention.
A TTI not requiring activation is disclosed by U.S. Pat. No. 4 448,548. It is designed for the temperature region used in steam sterilization of medical devices (121.degree. C. to 134.degree. C.) and its functionality is based on the liquification of a contained pellet of fusible material and its subsequent movement in a paper wick. Despite having a large size and a long wick (about 2 inches) the device is capable of indicating just one sterilization cycle. The device is incapable of indicating the multiplicity of sterilization cycles that a reuseable medical will undergo.
One deficiency of wick-based indicators is that for a wick to function effectively, i.e., as an absorbant medium for a fusible pellet of material, it must be of a size that is often too large for facile attachment to re-sterilizable medical devices. There exists a need for steam sterilization TTI devices that may be attached to, or be incorporated directly into the body of the reusable medical device. As will be disclosed below this may be accomplished by eliminating fusible materials and the porous wick.
There exists a need for TTI's, without fusible materials and absorbant wicks, that can reliably indicate and differentiate between the time elements corresponding to as few as one and as many as 100 high temperature sterilization cycles. The intended temperature range for these devices lies between 100.degree. C. and 160.degree. C.